The present invention relates to a liquid crystal display device that employs a thin film transistor array substrate provided with a supplementary capacitance for each pixel.
In recent years, as a thin type light-weight display having a low power consumption, an active matrix type liquid crystal display device in which each pixel electrode is controlled by a semiconductor element such as a thin film transistor provided for each pixel electrode has attracted a great deal of attention among, in particular, liquid crystal display devices, since the device can obtain an excellent resolution and a vivid image. Reference will be made below to the active matrix type liquid crystal display device.
As a semiconductor element for use in the conventional active matrix type liquid crystal display device, a thin film transistor constructed of an amorphous silicon thin film has been known, and a great many active matrix type liquid crystal display devices mounted with the thin film transistors are currently produced on the commercial basis. The active matrix type liquid crystal display device is about to go mainstream as a display for Office Automation equipment and commercial equipment.
With regard to the active matrix type liquid crystal display device, a transmission type liquid crystal display device that employs a transparent conductive thin film of ITO (Indium Tin Oxide) or the like for pixel electrodes is general. In the active matrix type liquid crystal display device, a capacitor is constructed by holding a liquid crystal layer between the pixel electrode and an opposite electrode provided on the opposite substrate side, and display is effected by retaining the electric potential of the pixel electrode at a predetermined voltage for a specified period in correspondence with an image signal. However, the display may be deteriorated by the potential reduction of the pixel electrode due to the electric discharge of the capacitor ascribed to a leak current or the like when the pixel use thin film transistor is turned off. It is sometimes the case where the pixel potential might fluctuate under the influence of the potentials of the neighbor wiring. Accordingly, in order to prevent the above-mentioned potential fluctuation of the pixel electrode, a supplementary capacitance is normally formed parallel to the capacitor.
The conventional supplementary capacitance has often been formed by using a gate insulating film as a dielectric film for forming the supplementary capacitance, using a capacitive wiring formed on a layer identical to that of a gate wiring or the gate wiring as one electrode and holding the dielectric film between the one electrode and a drain electrode or a pixel electrode. The reason for the above is that the dielectric film can be formed concurrently with the fabrication of the thin film transistor and that a gate insulating film having a good quality can be utilized for the dielectric film.
As a liquid crystal display device in which the supplementary capacitance is formed by the fabricating method described above, there is one shown in FIG. 7. As shown in FIG. 7, this liquid crystal display device has a polysilicon 52 that is formed on a transparent substrate 51 and patterned in an island-like shape, a gate insulating film 53 formed on the polysilicon 52, a gate electrode 54 and a common electrode 55 formed on the gate insulating film 53, a first interlayer insulating film 58 formed on the gate insulating film 53, the gate electrode 54 and the common electrode 55, a drain electrode 56 and a source electrode 57 formed on the first interlayer insulating film 58, a second interlayer insulating film 59 formed on the first interlayer insulating film 58, the drain electrode 56 and the source electrode 57, a transparent conductive film 61 formed on the second interlayer insulating film 59 and a pixel electrode 62 that is formed on the transparent conductive film 61 and is electrically connected to the drain electrode 56 at a contact hole 60. The supplementary capacitance is formed by using the gate insulating film 53 as a dielectric film and holding the gate insulating film 53 between the common electrode 55 and the polysilicon 52 (a region located on the connected drain electrode 56 side).
However, the aforementioned liquid crystal display device uses the gate insulating film 53 directly as a dielectric film for forming the supplementary capacitance. Therefore, although the fabricating method becomes relatively simple, it is often the case where the film thickness and the like of the gate insulating film are restricted to a certain extent in order to assure the performance of the thin film transistor. Due to this restriction, it has not been easy to concurrently assure the performances required for the gate insulating film and the dielectric film. Also, the capacitive wiring is formed in the same layer as that of the gate wiring, and therefore, it is difficult to secure a capacitive electrode area for forming a sufficient supplementary capacitance in assuring the processing accuracy of a photolithographic process and an etching process or the aperture rate of the pixel. The above fact has tended to be more significant as the processing dimensions of the thin film transistor become smaller, i.e., as the panel has a higher resolution with the dimensional reduction of the pixels. As described above, the conventional liquid crystal display device has the problem that the formation of the sufficient supplementary capacitance is very hard to achieve according to the improvement in the resolution of the liquid crystal display device.
Furthermore, the liquid crystal display device generally receives the influence of the electric fields of the adjacent electrodes at the boundaries of the pixel electrodes and the influence of the electric field of the bus line. This might cause the display of an image different from the image intended to be displayed on the pixel or the leak of light due to the absence of an electric field. Therefore, it is not appropriate to use the boundary portions of mutually adjoining pixel electrodes for display, and normally these portions are shielded by a black matrix. Conventionally, the black matrix has been required to be constructed of another layer. For example, Japanese Patent Laid-Open Publication No. HEI 5-216067 and so on propose the use of the bus line as a black matrix. However, in practice, there is the problem that the display becomes unstable under the influence of a bus line signal exerted on the pixel electrodes.